Patent literature 1 teaches allocating a real page of a tier-managed pool to a virtual page of a virtual volume in accordance with a write request, and migrating the real page allocated to the virtual page in accordance with the access condition to the virtual page among the tiers within the pool.
Patent literature 2 teaches migrating the data stored in a real area of a sequential access-type storage device allocated to a virtual volume to a real area of a random access-type storage device, and migrating the data stored in the real area of the random access-type storage device allocated to a virtual volume to the real area of the sequential access-type storage device, and after the migration is completed, changing the allocation relationship of the virtual volume and the real area.